The notion of a ‘microvalve’ rejects, on the surface, its use for high-flow applications in industrial process fluid distribution and control. Mass flow controllers (MFCs) in semiconductor processing require gas flows up to 50 standard liters per minute (slm). And, gas or liquid chromatography instruments can require similar sorts of flows. Refrigerators require liquid flows up to 12 cubic centimeters per minute (ccm), equivalent to perhaps 12 slm of gas flow. Manifold air flow applications in automobile engines also require high air flow to optimize vehicle motor efficiency and power. Requirements for control of high-purity fluids, proportional flow control, and for fast response, raise additional barriers to the use of microvalves in such high-flow applications.
U.S. Pat. No. 4,538,642 [1] and U.S. Pat. No. 4,585,209 [2] disclose an array of apertures in the inlet and/or the valve seat of a small valve. Electrostatic actuation of a diaphragm or cantilever against the array turns flow on and off. It is clear from the design that an array of apertures in the valve seat is used to provide mechanical support for the diaphragm or cantilever and to facilitate the electrostatic force for opening and closing. Neither (642) nor (209) discusses the benefits of flow through multiple apertures.
U.S. Pat. No. 5,333,831 [3] discusses raising the edges of the valve seat in a microvalve, in order to reduce the resistance to flow through a microvalve. This invention teaches the use of a microvalve with a single inlet and outlet.
Zdeblick [4] teaches the essential behavior and technology of a microvalve which uses thermopneumatic actuation. U.S. Pat. No. 4,966,646 is incorporated herein by reference.
Richter [5] describes a microvalve having a valve flap which actuates against a plurality of valve openings constituting valve seats. The structures disclosed include many different outlet port channel designs and opening designs; one design includes the teachings of [3], with respect to raising valve seat edges to minimize the flow resistance through the structure. The flow is described qualitatively as proportional to the plurality of valve openings and the distance to the valve flap. The Richter invention is a two-positioned valve, open or closed; there is no proportional control. And, as with previous inventions except Zdeblick's, the flow direction is vertical or transverse, passing through a “rigid” diaphragm and exiting through the plane of the valve seat.
Wang [6] describes an “order of magnitude” microvalve flow model, wherein the flow is linearly proportional to the gap between the rigid boss on a diaphragm, and the valve seat. As such, the model does not account for flows when the gap is relatively large compared to the diameter of the valve seat of the orifice. The effective area for flow is given quantitatively as the product of the sum of the four orifice sides and the gap distance. Wang appears to be attempting to use a simplified version of the ideal sonic flow equation for compressible gases.
U.S. Pat. No. 6,129,331 [7] reports a microvalve with a deformable membrane against a raised valve seat. ‘Deformable’ in this sense means the membrane is not necessarily flat, but can have a curvature defined by the thickness of the membrane, and the edge structure of the frame which supports the membrane. This reference also describes a microvalve which is surface mounted. That is, unlike the microvalve references above, the flow enters and exits the microvalve through a single plane, which is also the plane of attachment between the microvalve and its supporting package or manifold. Such a surface mounting scheme limits the materials wetted by the controlled fluid to the valve material, the attachment material and the manifold material. U.S. Pat. No. 6,129,331 is incorporated herein by reference.
van der Wijngaart [8] reports finite element simulation of microvalve flow. Various valve seat topologies are studied. Each is configured as an array of square orifices, some with raised edges. A complex diaphragm, with inlet holes to allow flow through the diaphragm, is raised and lowered uniformly toward or away from the valve seat array. That is, the diaphragm is not deformable. An alternate design has the flow entering from the side and raised bosses on the diaphragm act to seal against the valve seat. van der Wijngaart recognizes that as the diaphragm or boss approaches the valve seat, the flow, through the orifice in the valve seat, becomes a function of the area defined by the boss perimeter and the gap, z. When the boss is more than r/2 away from an orifice of radius r, the orifice area becomes the critical parameter in determining flow. van der Wijngaart focuses on short stroke valves, much less than 50 μm. He is trying to solve the problem of achieving flows greater than can be supported by an orifice whose radius is one-half the stroke length. van der Wijngaart's solution is to make multiple orifices of the maximum allowed diameter while staying just inside the “valve seat” control region or r/2. Nowhere does he mention the importance of the orifice periphery.
All microvalves in the prior art are limited in flow to 5 slm or less. There is a need for a microvalve which can achieve higher flows.